Various methods of handover (Inter-RAT Handover) between multiple radio communication systems using different radio access technologies (RATs), such as a 3G (Wideband-CDMA) system (hereinafter 3G) and a Long Term Evolution system (hereinafter LTE), are specified for a mobile station connectable to the multiple radio communication systems (for example, Non-patent Document 1).
There is control called Redirection performed when a mobile station switches a radio communication system to which the mobile station is connected between such radio communication systems of different RATs. Redirection specifies a switching target radio communication system (RAT) and switching target frequencies. The mobile station searches for cells of the frequencies of the radio communication system thus specified, in accordance with the RAT used by the radio communication system.
During Redirection, a connection between the mobile station and a base station of a switching source radio communication system is in a disconnected state. Specifically, the mobile station first disconnects the connection to the switching source radio communication system, and then, establishes connection to the switching target radio communication system and issues a connection request to the radio communication system. For Redirection from 3G to LTE, in particular, the specification (RRC CONNECTION REJECT) of executing Redirection to LTE immediately in response to a communication start message (RRC CONNECTION REQUEST) is specified (see Chapter 8.1.3.9 of Non-patent Document 1).
According to the above specification, switching to LTE is executed before a core network acquires the capability of the mobile station such as a frequency band in which the mobile station is capable of communications. This enables prompt switching of a radio communication system.